The present invention relates to an electrostatic driving apparatus for a microactuator, and more particularly, to an electrostatic driving apparatus for a microactuator which is capable of providing the microactuator highly sensitive performance by offsetting parasitic capacitances against each other.
A micromechanical resonating actuator has been an essential element of a highly sensitive sensor for converting potential or kinetic energy. Particularly, a microgyroscope capable of detecting an angular velocity using a resonator as well as a sensor for sensing pressure, acceleration and gas distribution based on a change in the resonant frequency of a resonator having extremely low attenuation has been actively researched.
FIG. 1 is a circuit diagram of an electrostatic driving apparatus for a conventional comb type microactuator. As shown in FIG. 1, the comb-driven type microactuator includes a combed activating vibration plate 1 fixed by a fixing portion 11a, a combed sensing plate 2 fixed by a fixing portion 12a, a combed suspended vibration plate 3 supported by a support beam 3a and a fixing portion 3b, a ground plate 4 grounded by a ground electrode 4a, a sensing unit 6, and a power unit 5. Here, the power unit 5 supplies AC and DC power to the activating vibration plate 1 via the fixing portion 11a.
If power is supplied to the activating vibration plate 1 from the power unit 5, an electrostatic force is generated between the teeth of the combed activating vibration plate 1 and the teeth of the suspended vibration plate 3, so that the suspended vibration plate 3 vibrates at a resonant frequency. Here, the sensing unit 6 senses a change in the capacitance according to a change in the opposite area opposing areas between the teeth of the sensing plate 2 and the teeth of the suspended vibration plate 3, as a voltage.
FIG. 2 is a diagram of an electrostatic driving apparatus a conventional parallel plate type microactuator. As shown in FIG. 2, the parallel plate type microactuator has a sensing plate 12 at the center thereof, and activating vibration plates 11 at both sides of the sensing plate, and a suspended vibration place 13 over the activating vibration plates 11 and the sensing plate 12. Here, in the circuit of the microactuator, a sensing unit 16 is connected to the sensing plate 12. Also, there is a power unit 15 providing AC and DC power.
If the power is supplied to the activating vibration plate 11 from the power unit 15, an electrostatic force is generated between the activating vibration plate 11 and the suspended vibration plate 13, so that the suspended vibration plate 13 vibrates up and down at a resonant frequency. Here, the sensing unit 16 senses a change in the capacitance according to a change in distance between the sensing plate 12 and the suspended vibration plate 13, as a voltage.
The vibration characteristics of the above electrostatic driven microactuator can be understood easily using an optical unit such as a laser interferometer. However, a simple circuit capable of detecting a change in the capacitance according to the vibration is necessary to apply the vibration characteristics to a sensor. However, since the vibration displacement of a resonator is very small, a change in a signal according to a change in the capacitance to be sensed is very small, so that it is difficult for the vibration characteristics to be applied to sense the change in the signal. To solve this problem, generally, the actuator and a sensing circuit are integrated or the displacement is sensed by a complicated signal processing method. However, there is a limitation to improvement of the signal-to-noise ratio, and the initial costs are high due to an additional required device. Thus, it is necessary to develop a simple sensing device of an off-chip type or an open-loop type.
As shown in FIG. 3, which is section view of FIG. 1, when intending to sense the displacement of a moving resonator 3 (vibration plate) while resonating a vibration plate formed on a silicon substrate, parasitic capacitance 10 and 17 exists between the activating vibration plate 1 generating a vibration signal and the sensing plate 2 detecting a sensing signal, forming a path for transmitting signals through a substrate 14 and each electrode as shown in FIG. 3. Such path for transmitting noise signals acts as a path for transferring noise, so that the activation vibration signal is transferred to the sensing unit via the parasitic path, and the vibration signal is mixed with the sensing signal. Here, resistor R repersents an equivalent resistance in a substrate. Due to such parasitic capacitance, the activating vibration signal mixed with the sensing signal acts as a noise source which lowers the sensitivity in the detection of a sensing signal.
The noise problems due to parasitic capacitance will be described in detail with reference to FIG. 3.
Generally, the activating vibration signal for applying an electrostatic force on an electrostatic type actuator includes both DC and AC components. Here, the amplitude of the applied electrostatic force is proportional to the square of the applied signal, so that the following formulas are obtained. ##EQU1##
That is, the electrostatic force generated when applying an activating vibration signal to the actuator includes DC components, and frequency components of .omega. and 2.omega.. Thus, if a voltage V.sub.D (t) is applied to the actuator having low attenuation, a displacement of which frequency includes the .omega. component and amplitude is proportional to the product of DC and AC components. Here, the electrostatic capacitance to be sensed is as follows. EQU C(t)=C.sub.0 for no resonance EQU C(t)=C.sub.0 +.DELTA.C(t) for resonance EQU .DELTA.C(t)=.vertline.C.sub.m .vertline. sin.omega..sub.r t
Here, C.sub.0 represents the total electrostatic capacitance of a sensing unit in a stopped state, .vertline.C.sub.m .vertline. represents the change in the electrostatic capacitance, that is, amplitude, and .omega..sub.r represents the original vibration frequency of the vibration type actuator. Here, the output activating vibration signal, as noise, is transferred via the path for the parasitic capacitance of FIG. 3, so that the activating vibration signal is proportional to the AC component V.sub.ac, and then presents itself at a sensed output. Thus, in general, a high V.sub.dc, and a low V.sub.ac are applied, if possible, in order to increase the signal-to-noise ratio.
However, it is impossible to lower V.sub.ac, limitlessly in actual circumstances. Also, even though the level of V.sub.ac is lowered to a predetermined level, a considerable noise component which is proportional to V.sub.ac exists. Also, as V.sub.dc increases, the position of the vibration plate is shifted to one side from the center, by the DC component. As a result, the vibration characteristics of the vibration plate is changed, so that it is difficult to measure the resonant frequency.
According to the conventional electrostatic type microactuator, a parasitic capacitance exists between a port for applying an activating vibration signal to generate electrostatic force, and a port for sensing the change of capacitance, so that the activating vibration signal is transferred to the sensing port as noise. Thus, a signal generated by only the real displacement of the suspended vibration plate cannot be sensed precisely.